The invention relates generally to semiconductor integrated circuits, and more particularly, to an apparatus and method for ensuring that a device such as a spherical-shaped semiconductor integrated circuit is uniformly coated with processing material.
Typically, conventional integrated circuits, or "chips," are formed from a flat surface semiconductor wafer. The semiconductor wafer is first manufactured in a semiconductor material manufacturing facility and is then provided to a fabrication facility. At the latter facility, several layers are processed onto the semiconductor wafer surface. Once completed, the wafer is then cut into one or more chips and assembled into packages. Although the processed chip includes several layers fabricated thereon, the chip still remains relatively flat.
One processing step that is performed in the fabrication facility is photolithography. Photolithography requires a coating of photo resist material to be placed over the top wafer surface. Additional processing steps may also require that a coating of fluid be placed over the top wafer surface. With traditional flat wafers, these steps are performed in a relatively straight-forward manner, such as by pouring or spraying the fluid onto the wafer and the spinning the wafers to spread the resist.
More particularly, in order to spread the resist evenly across the surface of the wafer to achieve a thickness uniformity to within 2% to 10%, a certain angular velocity in revolutions per minute (RPM) is applied. This will exert a centrifugal force on the liquid resist material while the wafer itself remains horizontal. With conventional wafers, an RPM on the order of approximately 3000 to 4000 will give good results, depending on the wafer size and thickness of the coating desired.
This conventional process invokes both a tangential (until equilibrium is reached) as well as a centrifugal force on the fluid across the wafer surface. This process can be viewed in terms of surface velocity required to shift the liquid resist material uniformly over the surface of the wafer. For conventional two-dimensional (2D) wafers, this process is adequate.
However, a spherical substrate is three-dimensional (3D) and includes three axes of linear movement and three axes of rotational movement, thereby defining six "degrees of freedom." The linear axes represent vibrational movements (resonances) and the three rotational axes represent spin. It can easily be seen that applying the conventional process to distribute liquid resist material along the surface of a wafer is unworkable when applied to a spherical substrate. More particularly, if the spherical substrate is spun on a single axis, different thickness will be produced around the elevation angle of the spin axis, resembling Saturn's rings. Thus, it would be necessary to design equipment to spin the spherical substrate on multiple axes. While multiple spin axes equipment can be designed, a problem exists in that the equipment must contact the sphere.
One method of applying liquid material is by dropping the sphere through a "bubble" of resist material (surface tension method). While the surface tension method produces a consistent thickness on at least one-half of the spherical substrate (typically the front half that originally contacts the material) there tends to be some surface portions that have back-splashed resist onto the back side of the sphere. Those skilled in the art will understand that even slight uneven surface features of the resist coating will cause problems when the resist material is exposed to light during the exposure step of lithography. Typically, this inconsistent coating must be reworked, which is a very expensive process step in the fabrication of integrated circuits.
Therefore, it can be seen that a need exists for an apparatus and method of coating a spherical substrate with a uniform layer of photo-resist material.